Re: [syzbot] BUG: Bad page map (5)

[Yang Shi <shy828301@gmail.com>]

On Sun, Sep 11, 2022 at 9:27 PM syzbot
<syzbot+915f3e317adb0e85835f@syzkaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following issue on:
>
> HEAD commit:    e47eb90a0a9a Add linux-next specific files for 20220901
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=17330430880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=7933882276523081
> dashboard link: https://syzkaller.appspot.com/bug?extid=915f3e317adb0e85835f
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13397b77080000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1793564f080000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+915f3e317adb0e85835f@syzkaller.appspotmail.com
>
> BUG: Bad page map in process syz-executor198  pte:8000000071c00227 pmd:74b30067
> addr:0000000020563000 vm_flags:08100077 anon_vma:ffff8880547d2200 mapping:0000000000000000 index:20563
> file:(null) fault:0x0 mmap:0x0 read_folio:0x0
> CPU: 1 PID: 3614 Comm: syz-executor198 Not tainted 6.0.0-rc3-next-20220901-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  print_bad_pte.cold+0x2a7/0x2d0 mm/memory.c:565
>  vm_normal_page+0x10c/0x2a0 mm/memory.c:636
>  hpage_collapse_scan_pmd+0x729/0x1da0 mm/khugepaged.c:1199
>  madvise_collapse+0x481/0x910 mm/khugepaged.c:2433
>  madvise_vma_behavior+0xd0a/0x1cc0 mm/madvise.c:1062
>  madvise_walk_vmas+0x1c7/0x2b0 mm/madvise.c:1236
>  do_madvise.part.0+0x24a/0x340 mm/madvise.c:1415
>  do_madvise mm/madvise.c:1428 [inline]
>  __do_sys_madvise mm/madvise.c:1428 [inline]
>  __se_sys_madvise mm/madvise.c:1426 [inline]
>  __x64_sys_madvise+0x113/0x150 mm/madvise.c:1426
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7f770ba87929
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f770ba18308 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 00007f770bb0f3f8 RCX: 00007f770ba87929
> RDX: 0000000000000019 RSI: 0000000000600003 RDI: 0000000020000000
> RBP: 00007f770bb0f3f0 R08: 00007f770ba18700 R09: 0000000000000000
> R10: 00007f770ba18700 R11: 0000000000000246 R12: 00007f770bb0f3fc
> R13: 00007ffc2d8b62ef R14: 00007f770ba18400 R15: 0000000000022000
>  </TASK>

I think I figured out the problem. The reproducer actually triggered
the below race in madvise_collapse():

             CPU A
        CPU B
mmap 0x20000000 - 0x21000000 as anon

       madvise_collapse is called on this area

           Retrieve start and end address from the vma (NEVER updated
later!)

           Collapsed the first 2M area and dropped mmap_lock
Acquire mmap_lock
mmap io_uring file at 0x20563000
Release mmap_lock

            Reacquire mmap_lock

            revalidate vma pass since 0x20200000 + 0x200000 >
0x20563000

            scan the next 2M (0x20200000 - 0x20400000), but due to
whatever reason it didn't release mmap_lock

            scan the 3rd 2M area (start from 0x20400000)

              actually scan the new vma created by io_uring since the
end was never updated

The below patch should be able to fix the problem (untested):

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5f7c60b8b269..e708c5d62325 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2441,8 +2441,10 @@ int madvise_collapse(struct vm_area_struct
*vma, struct vm_area_struct **prev,
                memset(cc->node_load, 0, sizeof(cc->node_load));
                result = hpage_collapse_scan_pmd(mm, vma, addr, &mmap_locked,
                                                 cc);
-               if (!mmap_locked)
+               if (!mmap_locked) {
                        *prev = NULL;  /* Tell caller we dropped mmap_lock */
+                       hend = vma->end & HPAGE_PMD_MASK;
+               }

                switch (result) {
                case SCAN_SUCCEED:


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